Gaseous electron multiplier gain characteristics using low-pressure Ar/CO<Subscript>2</Subscript>     

T. Rogers  R. McEntaffer  T. Schultz  J. McCoy  D. Miles  J. Tutt 《Experimental Astronomy》2017,43(2):201-210

Gaseous Electron Multiplier detectors, or GEMs, show promise for use on space-based X-ray missions. Operating pressure strongly affects the gain of the detector and must be optimized for best performance. We have measured the gain characteristics of a GEM detector at various pressures below atmosphere using a mixture of Ar:CO₂ with the goal of maximizing gain to push GEM capabilities to the lowest energies possible. This paper discusses our tests, results, and their implications for choosing a detector pressure. We found that at any operating pressure the detector voltage can be adjusted to achieve roughly the same maximum gain prior to the onset of electrical discharges. We also find that the gain varies substantially by spatial location across the detector, but this variation is insensitive to changes in pressure allowing it to be calibrated and corrected if necessary. The detector pressure can therefore be optimized in the interest of other performance parameters such as leak rate, window stress, power requirements, or quantum efficiency without concern for negatively affecting the gain. These results can inform the choice of operating pressure and voltage for GEMs used onboard future space missions.  相似文献   

Simulating Groundwater-Streamflow Connections in the Upper Colorado River Basin     

Hoang Tran  Jun Zhang  Jean-Martial Cohard  Laura E. Condon  Reed M. Maxwell 《Ground water》2020,58(3):392-405

In mountain, snow driven catchments, snowmelt is supposed to be the primary contribution to river streamflows during spring. In these catchments the contribution of groundwater is not well documented because of the difficulty to monitor groundwater in such complex environment with deep aquifers. In this study we use an integrated hydrologic model to conduct numerical experiments that help quantify the effect of lateral groundwater flow on total annual and peak streamflow in predevelopment conditions. Our simulations focus on the Upper Colorado River Basin (UCRB; 2.8 × 10⁵ km²) a well-documented mountain catchment for which both streamflow and water table measurements are available for several important sub-basins. For the simulated water year, our results suggest an increase in peak flow of up to 57% when lateral groundwater flow processes are included—an unexpected result for flood conditions generally assumed independent of groundwater. Additionally, inclusion of lateral groundwater flow moderately improved the model match to observations. The correlation coefficient for mean annual flows improved from 0.84 for the no lateral groundwater flow simulation to 0.98 for the lateral groundwater flow one. Spatially we see more pronounced differences between lateral and no lateral groundwater flow cases in areas of the domain with steeper topography. We also found distinct differences in the magnitude and spatial distribution of streamflow changes with and without lateral groundwater flow between Upper Colorado River Sub-basins. A sensitivity test that scaled hydraulic conductivity over two orders of magnitude was conducted for the lateral groundwater flow simulations. These results show that the impact of lateral groundwater flow is as large or larger than an order of magnitude change in hydraulic conductivity. While our results focus on the UCRB, we feel that these simulations have relevance to other headwaters systems worldwide.  相似文献   

Characterizing beach intertidal bar systems using multi-annual LiDAR data     

Andrew Miles  Suzana Ilic  Duncan Whyatt  Mike R. James 《地球表面变化过程与地形》2019,44(8):1572-1583

Intertidal bars are common in mesotidal/macrotidal low-to-moderate energy coastal environments and an understanding of their morphodynamics is important from the perspective of both coastal scientists and managers. However, previous studies have typically been limited by considering bar systems two-dimensionally, or with very limited alongshore resolution. This article presents the first multi-annual study of intertidal alongshore bars and troughs in a macrotidal environment using airborne LiDAR (light detection and ranging) data to extract three-dimensional (3D) bar morphology at high resolution. Bar and trough positions are mapped along a 17.5 km stretch of coastline in the northwest of England on the eastern Irish Sea, using eight complete, and one partial, LiDAR surveys spanning 17 years. Typically, 3–4 bars are present, with significant obliquity identified in their orientation. This orientation mirrors the alignment of waves from the dominant south-westerly direction of wave approach, undergoing refraction as they approach the shoreline. Bars also become narrower and steeper as they migrate onshore, in a pattern reminiscent of wave shoaling. This suggests that the configuration of the bars is being influenced by overlying wave activity. Net onshore migration is present for the entire coastline, though rates vary alongshore, and periods of offshore migration may occur locally, with greatest variability between northern and southern regions of the coastline. This work highlights the need to consider intertidal bar systems as 3D, particularly on coastlines with complex configurations and bathymetry, as localized studies of bar migration can overlook 3D behaviour. Furthermore, the wider potential of LiDAR data in enabling high-resolution morphodynamic studies is clear, both within the coastal domain and beyond. © 2019 John Wiley & Sons, Ltd.  相似文献   

一种新的土壤冻融特征曲线模型   总被引：1，自引：0，他引：1  

付子腾  吴青柏  Miles DYCK  何海龙 《冰川冻土》2021,43(2):437-452

土壤冻融特征曲线(SFTC)可以描述冻融过程中未冻水含量随负温的变化关系.准确刻画土壤冻融特征曲线对土壤的冻融过程及相关的水热耦合运移研究至关重要.以往研究中土壤冻融特征曲线适用范围较窄,无法满足实际需要.通过类比水分特征曲线,考虑初始含水量和溶质浓度对未冻水含量的影响,提出了一种新的冻融特征曲线模型,经广泛的文献数据...  相似文献   

An assessment of the effect of urban development on groundwater levels in a chalk aquifer     

J. C. Miles  J. W. Lloyd  J. A. J. Witt 《Environmental Geology》1989,14(3):187-194

Underlying the site of a proposed development is a chalk aquifer, groundwater levels are close to the surface. The storm water from the development is to be drained into a lake in the center of the development. Because of the high groundwater levels the effect of this lake has been studied by means of a simple localized mathematical model. It is shown that such a model can give results of sufficient accuracy with regard to probable changes in groundwater levels caused by the development.  相似文献   

Development and parameterisation of a complex hydrogeological model based on high-resolution direct-push data     

B. Miles  T. Kalbacher  O. Kolditz  C. Chen  J. Gronewold  W. Wang  A. Peter 《Environmental Geology》2007,52(7):1399-1412

Ongoing developments in geological and hydrogeological investigation techniques, especially direct-push methods, have led to an increase in the quality, density and spatial resolution of data available from such investigations. This has created new challenges in the development of numerical models in terms of accurately and efficiently translating detailed and complex conceptual models into effective numerical models. Suitable geometrical and numerical modelling tools are essential in order to meet these challenges. This paper describes the development of a three-dimensional hydrogeological flow model for a contaminated site near Berlin, Germany, based on high-resolution geological data obtained principally using direct-push methods. The available data were first interpreted to construct a detailed GIS-based geological model, which formed the basis of the conceptual site model. The conceptual model was then translated into a geometrical model, which was used to create a finite element numerical model. An innovative geometry object-based approach enabled the complex structural details of the conceptual model to be accurately reproduced in the numerical model domain. The resulting three-dimensional steady-state unconfined flow model was successfully calibrated using external automated calibration software, whereby parameter values for groundwater recharge and hydraulic conductivity were determined.  相似文献   

Predicting the future carbon budget of an upland peat catchment     

Fred Worrall  Tim Burt  John Adamson  Mark Reed  Jeff Warburton  Alona Armstrong  Martin Evans 《Climatic change》2007,85(1-2):139-158

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AN L6 CHONDRITE FOUND NEAR PARACHILNA,SOUTH AUSTRALIA*     

S.J.B. Reed  D.H. McColl 《Meteoritics & planetary science》1975,10(3):227-232

An 8.82 kg stony meteorite found on Motpena Station, 31°6' S, 138° 16' E, near Parachilna, South Australia, is shown to be a somewhat weathered L6 (olivine-hypersthene) chondrite. This is listed in The Meteoritical Bulletin, No. 53, June 1975 as MOTPENA, Synonym, Parachilna.  相似文献   

Pattern and process of land loss in the Mississippi Delta: A Spatial and temporal analysis of wetland habitat change   总被引：2，自引：0，他引：2  

John W. Day  Louis D. Britsch  Suzanne R. Hawes  Gary P. Shaffer  Denise J. Reed  Donald Cahoon 《Estuaries and Coasts》2000,23(4):425-438

An earlier investigation (Turner 1997) concluded that most of the coastal wetland loss in Louisiana was caused by the effects of canal dredging, that loss was near zero in the absence of canals, and that land loss had decreased to near zero by the late 1990s. This analysis was based on a 15-min quadrangle (approximately 68,000 ha) scale that is too large to isolate processes responsible for small-scale wetland loss and too small to capture those responsible for large-scale loss. We conducted a further evaluation of the relationship between direct loss due to canal dredging and all other loss from 1933–1990 using a spatial scale of 4,100 ha that accurately captures local land-loss processes. Regressions of other wetland loss on canal area (i.e., direct loss) for the Birdfoot, Terrebonne, and Calcasieu basins were not significant. Positive relationships were found for the Breton (r²=0.675), Barataria (r²=0.47), and Mermentau (r²=0.35) basins, indicating that the extent of canals is significantly related to wetland loss in these basins. A significant negative relationship (r²=0.36) was found for the Atchafalaya coastal basin which had statistically lower loss rates than the other basins as a whole. The Atchafalaya area receives direct inflow of about one third of the Mississippi discharge. When the data were combined for all basins, 9.2% of the variation in other wetland loss was attributable to canals. All significant regressions intercepted the y-axis at positive loss values indicating that some loss occurred in the absence of canals. Wetland loss did not differ significantly from the coast inland or between marsh type. We agree with Turner that canals are an important agent in causing wetland loss in coastal Louisiana, but strongly disagree that they are responsible for the vast majority of this loss. We conclude that wetland loss in the Mississippi delta is an ongoing complex process involving several interacting factors and that efforts to create and restore Louisiana’s coastal wetlands must emphasize riverine inputs of freshwater and sediments.  相似文献   

Plankton: A guide to their ecology and monitoring for water quality, 2nd edition     

Miles Lamare 《新西兰海洋与淡水研究杂志》2019,53(3)

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